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Message-Id: <20090731091744.B6DE.A69D9226@jp.fujitsu.com>
Date: Fri, 31 Jul 2009 15:47:55 +0900 (JST)
From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To: David Rientjes <rientjes@...gle.com>
Cc: kosaki.motohiro@...fujitsu.com,
Andrew Morton <akpm@...ux-foundation.org>,
Rik van Riel <riel@...hat.com>,
Paul Menage <menage@...gle.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [patch -mm v2] mm: introduce oom_adj_child
Hi
> On Thu, 30 Jul 2009, KOSAKI Motohiro wrote:
>
> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -426,7 +426,7 @@ static struct mm_struct * mm_init(struct mm_struct * mm, struct task_struct *p)
> > > init_rwsem(&mm->mmap_sem);
> > > INIT_LIST_HEAD(&mm->mmlist);
> > > mm->flags = (current->mm) ? current->mm->flags : default_dump_filter;
> > > - mm->oom_adj = (current->mm) ? current->mm->oom_adj : 0;
> > > + mm->oom_adj = p->oom_adj_child;
> >
> > This code doesn't fix anything.
> > mm->oom_adj assignment still change vfork() parent process oom_adj value.
> > (Again, vfork() parent and child use the same mm)
> >
>
> That's because the oom killer only really considers the highest oom_adj
> value amongst all threads that share the same mm. Allowing those threads
> to each have different oom_adj values leads (i) to an inconsistency in
> reporting /proc/pid/oom_score for how the oom killer selects a task to
> kill and (ii) the oom killer livelock that it fixes when one thread
> happens to be OOM_DISABLE.
I agree both. again I only disagree ABI breakage regression and
stupid new /proc interface.
Paul already pointed out this issue can be fixed without ABI change.
> So, yes, changing the oom_adj value for a thread may have side-effects
> on other threads that didn't exist prior to 2.6.31-rc1 because the oom_adj
> value now represents a killable quantity of memory instead of a being a
> characteristic of the task itself. But we now provide the inheritance
> property in a new way, via /proc/pid/oom_adj_child, that gives you all the
> functionality that the previous way did but without the potential for
> livelock.
maybe, I should say my stand-point obviously. I don't dislike your
per-process oom_adj concept.
I only oppose vfork breakage.
if you feel my stand point is double standard, I need explain me more.
So, I don't think per-process oom_adj makes any regression on _real_ world.
but vfork()'s one is real world issue.
I think they are totally different thing.
And, May I explay why I think your oom_adj_child is wrong idea?
The fact is: new feature introducing never fix regression. yes, some
application use new interface and disappear the problem. but other
application still hit the problem. that's not correct development style
in kernel.
>
> > IOW, in vfork case, oom_adj_child parameter doesn't only change child oom_adj,
> > but also parent oom_adj value.
>
> Changing oom_adj_child for a task never changes oom_adj for any mm, it
> simply specifies what default value shall be given for a child's newly
> initialized mm. Chaning oom_adj, on the other hand, will
Ah, ok. I miunderstood.
However, We can fix this issue without new interface, isn't it?
> > IOW, oom_adj_child is NOT child effective parameter.
> >
>
> It's not meant to be, it's only meant to specify a default value for newly
> initialized mm's of its descendants. What happens after that is governed
> completely by the child's own /proc/pid/oom_adj. That's pretty clearly
> explained in Documentation/filesystems/proc.txt.
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